How to automate polygon reduction using python?
I'd recommend using instant-meshes with it's batch mode. You could write a python script like this one to do all the files in your folder:

import subprocess
import os

models_folder = 'C:/folder'
models = sorted(os.listdir(models_folder))

output_folder = 'C:/output_folder'

instant_meshes_path = 'C:/Instant_Meshes.exe'

for file in models:
    # Desired number of faces for the model
    faces = 2500

    subprocess.call([
        instant_meshes_path,
        '--faces', str(faces),
        os.path.join(models_folder, file),
        os.path.join(output_folder, file)
    ])